1. Field of Invention
The present invention relates to a method of improving adhesion between flexible polyvinyl chloride sheet material and polyurethane foam.
2. Background Discussion and Related Art
The present invention relates to composite plastic moldings having a solid, thin skin of polyvinyl chloride (PVC) and a foamed backing layer and a process for their production. Such moldings are widely used in the manufacture of interior pans for motor vehicles, such as instrument panels, center consoles, arm rests or door inner panels. They also find utility in furniture, toys and a number of other applications well known in the art.
In the production of such composite plastic moldings, the solid PVC skin is generally produced in a known manner from a thermoplastic in a first step. This step is carried out in a suitable mold, for example by slush molding from a PVC-plasticizer plastisol or a PVC-powder-containing polymeric plasticizer.
In a second step, the solid skin produced in the first step is back-foamed with a suitable plastic either in the same mold (deep drawing) or after transfer to a second mold (slush molding). In automobile applications, the back-foaming can be done in place by injecting the suitable plastic between the flexible PVC skin and a rigid backing that the PVC covers. By virtue of their excellent foamability and the range of variation of the physical properties of the foam, polyurethane(urea) systems are generally used for this back-foaming step.
The polyurethane(urea) system, referred to hereafter as polyurethane (PU) foam, used for backing the PVC skin can be made from known processes and materials, such as polyisocyanates, polyols, catalyst and other additives (for example, blowing agents and cell stabilizers).
Suitable organic polyisocyanates include hexamethylene diisocyanate, phenylene diisocyanate, toluene diisocyanate and 4,4'-diphenylmethane diisocyanate. Especially suitable are mixtures of diisocyanates known commercially as "crude MDI", also known as PAPI, which contains about 60% of 4,4'-diphenylmethane diisocyanate along with other isomeric and analogous higher polyisocyanates. Also suitable are "prepolymers" of these polyisocyanates comprising a partially prereacted mixture of polyisocyanates and polyether or polyester polyols. Other suitable isocyanates are the 2,4- and 2,6-toluene diisocyanates ("TDI"), individually or together, in commercially available mixtures.
Illustrative of suitable polyols as a component of the PU foam are polyalkylene ether and polyester polyols. The polyalkylene ether polyols include the poly(alkylene oxide) polymers such as poly( ethylene oxide) and poly(propylene oxide) polymers and copolymers with terminal hydroxyl groups derived from polyhydric compounds including diols and triols. Examples include ethylene glycol, propylene glycol, 1,3-butane diol, 1,4-butane diol, 1,6-hexane diol, neopentyl glycol, diethylene glycol, dipropylene glycol, pentaerythritol, glycerol, diglycerol, trimethylol propane, cyclohexane diol and like low molecular weight polyols. Useful polyester polyols include those produced by reacting a dicarboxylic acid with an excess of a diol, for example, adipic acid with ethylene glycol or butane diol, or reacting a lactone with an excess of a diol such as caprolactone and propylene glycol.
Catalyst compositions typically used in the art include tertiary amines. The tertiary amines which may be protonated with the acid composition to form partial amine salts may be any of the tertiary amine compounds typically used as catalysts for the urethane reaction. Such tertiary amines are well known to those skilled in the art and would include, by way of example, pentamethyldiethyltriamine, trimethylaminoethyl ethanolamine, tetramethylamino-bis-propylamine, 2,6-diaza-2,6-dimethyl-10-hydroxydecane, N-ethyl morpholine, N,N'-dimethylpiperazine, N,N-dimethylaminopropylamine, triethylenediamine, N,N-dimethylaminoethoxyethanol, N,N-diethylaminoethoxyethanol, tetramethyl-2-hydroxypropyl diethyltriamine, dimethylaminoethanol, hydroxypropyl piperazine, bis-dimethylaminoethyl piperazine and N,2-hydroxypropyl morpholine.
Blowing agents include water, methylene chloride, trichlorofluoromethane and the like. Cell stabilizers include silicones.
The compounded PVC used in these applications can be externally or internally plasticized by methods well known in the art and can contain a wide selection of additives. Potential additives include (a) fillers and/or pigments (about 0% to about 20% by weight based on the PVC); (b) monomeric and polymeric process aids (about 2% to about 15% by weight); (c) lubricants (about 0.5% to about 3% by weight); (d) PVC stabilizers and antioxidants (about 3% to about 10% by weight); and (e) other functional additives (present in relatively minor amounts, typically under 2% by weight) including ultraviolet light stabilizers, drying agents, and the like.
Example components typically used in such compounded PVC include:
1. Resins such as acrylonitrile-butadiene-styrene copolymers, acrylic-rubber-modified styrene acrylonitrile copolymer and styrene-acrylonitrile polymer; PA1 2. Liquid plasticizers such as diundecyl phthalate, tri-2-ethylenehexyl trimellitate, diisononyl phthalate, and polyesters; PA1 3. Impact modifiers such as grafted particulate rubbery polymers such as methacrylate-butadiene-styrene (MBS), acrylate-methacrylate (all acrylic), acrylate-butadiene-methacrylate (modified acrylic), and acrylonitrile-butadiene-styrene (ABS); semicompatible plasticizing polymers such as chlorinated polyethylene (CPE) and ethylene-vinyl acetate (EVA); inorganics such as stearic acid-coated calcium carbonate; nitrile rubbers (NBR); and copolymers of vinyl chloride and insoluble rubber; PA1 4. Fillers and pigments (to provide color, opacity, and aid in the calendering of the PVC) such as titanium dioxide, calcium carbonate, zinc oxide, white lead, gypsum, precipitated silica, carbon black, and red iron oxide; PA1 5. Processing aids (to aid in the processing of the calenderable PVC and help in giving the final composite a favorable visual appearance) such as PVC/acrylate resin, acrylate processing aids, chlorinated polyethylene and alpha methyl styrene or acrylic polymers; PA1 6. Lubricants such as stearic acid, oxidized polyethylene wax and other low molecular weight polymers; and PA1 7. Stabilizers and antioxidants such as stearate salts, epoxidized vegetable oils, dibasic lead phosphite and dibutyl tin dilaurate, dimaleate or mercaptide.
In order to meet the rather stringent requirements regarding such combined characteristics as the pliability, strength and cold crack resistance needed in the product, the art has taught, in addition to the above, adding an effective amount of an ethylene/carbon monoxide terpolymer to the PVC compound. In particular, such properties as pliability, strength, low temperature flexibility or cold crack resistance, and color stability upon heat aging of the films are improved.
The ethylene/carbon monoxide polymer can be (as suggested in U.S. Pat. No. 3,780,140 to C. F. Hammer) used as an additive to conventional, noninternally-plasticized PVC and can be (as suggested in U.S. Pat. No. 4,489,193 to J. C. Goswami) used as an additive to internally-plasticized PVC. Both of these references are incorporated by reference. Useful ethylene/carbon monoxide polymers are copolymers of (a) from about 40% by weight (wt.%) to about 80 wt. % ethylene; (b) about 3 wt. % to about 30 wt. % carbon monoxide; and (c) from about 5 wt. % to about 60 wt. % of one or more termonomers copolymerizable therewith. Sulfur dioxide may be used in place of the carbon monoxide. Examples of suitable termonomers include the C.sub.3 -C.sub.20 unsaturated mono- and di-carboxylic acids and their esters; the C.sub.1 -C.sub.18 vinyl esters of saturated carboxylic acids; the vinyl C.sub.1 -C.sub.18 alkyl ethers, acrylonitrile; methacrylonitrile; the C.sub.3 -C.sub.12 alpha olefins, and ring compounds such as norbornene and vinyl aromatic compounds. A preferred polymer is of the form E/X/Y, having a melt index of 1.0 to 500 as measured using ASTM D-1238; wherein E is ethylene; X is a vinyl ether, vinyl acetate or an alkyl acrylate or methacrylate, the alkyl group having from 1 to 8 carbon atoms; and Y is carbon monoxide or sulfur dioxide, more preferably carbon monoxide. X preferably is 20-50 wt. %, more preferably 25-45 wt. % and most preferably 30-40 wt. % of the E/X/Y; and Y preferably is 5-40 wt. %, more preferably 7-25 wt. %, and most preferably 8-12 wt. % of the E/X/Y; with E being the remainder. Ethylene/vinyl acetate/carbon monoxide copolymer and ethylene/n-butyl acryl at e/carbon monoxide copolymer are preferred. The ethylene/carbon monoxide polymers of this type are commercially available under the trademark ELVALOY.RTM. from E. I. du Pont de Nemours and Company.
While addition of such ethylene/carbon monoxide polymers has improved the physical properties of the PVC skin, it has created a problem in producing the composite. In particular, when the PVC skin is back-foamed with the PU, the PU does not effectively adhere to the PVC. It has thus become necessary to apply an adhesive to the PVC prior to back-foaming with PU. This is a costly additional step. Also, if adhesion is not effective, the PVC skin can separate from the PU foam, particularly after aging at temperatures experienced in an automobile allowed to sit in the sun.